Among materials whose fluidity varies by light there is a photocurable resin that loses its fluidity when the liquid material is irradiated with light, resulting in solidification, and has found wide use as adhesives, coating agents or the like. Such a photocurable resin cures mainly as polymerization and crosslinking reactions proceed, and the curing reaction is irreversible due to the formation of chemical bonds involved.
Some reversible adhesives include a conventional hot-melt type adhesive, but do not have workability at room temperature because of being liquefied only upon heating. There have also been studies made of adhesives capable of breakdown by thermal decomposition or photo-decomposition after it has cured, but most break down only once.
Apart from this, sugar alcohol derivatives or the like having a polyvalent azobenzene structure have been proposed as an adhesive capable of reversible desorption by light (Patent Publications 1-4, and Non-Patent Publications 1-5). An adhesive comprising this material is capable of repeated adhesion and desorption over and over only by irradiation with light. This repeatability ensues from melting or softening point changes based on the repeatable photoisomerization of azobenzene with the result that a stable state at room temperature can alternate over and over between a liquid phase and a solid phase.
However, the use of an azo dye causes adhesives to be colored in yellow to orange, imposing limitation on the range of use. To obtain a colorless adhesive, it is required to change a photosensitive unit from azobenzene to a transparent one.
Among candidates for the photosensitive unit capable of reversible reactions there is colorless anthracene. It has been known that the anthracene is dimerized by photoreactions, but when heated, the linkage is cleaved off allowing it to return back to the original state. Light-induced phase transition making use of this has already been put forward (Patent Publications 5-10, Non-Patent Publications 6 and 7).
However, this material cannot take on a liquid state at room temperature. A problem with the material used as an adhesive or coating agent is that it must remain in a stable liquid state at room temperature.